This application claims the priority of German patent document 197 11 688.4 filed Mar. 20, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a knee protection system for motor vehicles, comprising an air bag module having an air bag which is arranged in the impact area of an occupant's knees and which is fixedly connected with the vehicle. Preferred embodiments of the invention provide such an airbag connected at two clamping points.
In a knee protection system of this type, called active knee protection, the air bag will inflate between the underpanelling and the occupant's knees in the event of a sudden deceleration of the vehicle. By inflating the air bag, the knee protection system is guided into the occupant so that he takes part as early as possible in the deceleration of the motor vehicle. In such an active knee protection system, the unfavorable flux of force of the air bag is a disadvantage. Immediately after activating the air bag module, the air bag must be inflated sufficiently tautly to prevent a hitting-through of the occupant's knees against the underpanelling of the motor vehicle. Since, among other things, the forces exercised on the air bag depend on the occupant's weight, it is virtually impossible to design the air bag module such that it causes an optimal deceleration for occupants of different weights.
Passive knee protection is also known. In passive knee protection, an energy-dissipating padding is rigidly installed in the vehicle in the impact areas of the knees. This padding depends on the shaping of the panelling parts and impairs the moving space. An important disadvantage is that because of the distance of the knees from the padding in the normal sitting position, the occupant only later participates in the deceleration of the vehicle, thereby resulting in higher stress to the occupant.
It is an object of the invention to further develop the active knee protection system of the above-mentioned type such that, independently of weight, the occupant is better protected in an accident.
According to the invention, this object is achieved in that the air bag is connected with the vehicle at at least one clamping point by a force limiting device. By means of a force limiting device, the deceleration due to the air bag is improved because the path available for decelerating the occupant's knees is increased. When, during a vehicle impact, the air bag module is activated and the air bag is inflated, the air bag will catch the occupant's knees while deforming itself. After a defined deceleration force is exceeded, the force limiting device becomes operative and permits a movement of the whole air bag under the applied stress.
The force limiting device is preferably an energy-dissipating element. The energy-dissipating element is preferably a plastically deformable element. In contrast to an energy-dissipating element based on friction, a plastically deformable element has the advantage that the course of the characteristic force path curve can be precisely defined.
The plastically deformable element may, for example, be constructed as an upset tube. A course of the characteristic force path curve which is particularly favorable can be achieved by a plastically deformable element constructed as an inverted tube or as a corrugated tube.
The air bag preferably comprises an enveloping fabric with mutually diagonally crossing threads. The enveloping fabric has the advantage that the air bag is shortened while it is inflating and, as a result, is stretched between the two clamping points.
For a slow deceleration, that is, for a soft catching of the occupant's knees, it is advantageous for the air bag to have a flow-out capability. In a serious accident, the air is first displaced from such an air bag by the occupant's impacting knees, whereby the knees are softly decelerated. When the damping capacity of the air bag has been used up in certain preferred embodiments, the force limiting device mounted at one or both clamping points becomes operative and permits an additional deceleration path.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.